Police divers have discovered the body of a four-year-old boy after the car he was a passenger in rolled backwards into the Hawkesbury River in Australia where it was washed away.

Emergency services rushed to a caravan park on River Road, Lower Portland, just before 11am after reports a car had rolled off the boat ramp and into the river.

A four-year-old boy was strapped into a back seat child seat when the car plunged into the water.

A NSW Police Force spokesman said it appeared several men were trying to reverse the vehicle, which was towing a boat trailer, down the boat ramp when the incident occurred.

Advertisement

"The car unfortunately contained a four-year-old child," he said.

"There's been no sign of the car."

The car was quickly submerged and washed downstream by the Hawkesbury's strong current.

The incident triggered a massive search of the area, including police divers, boat, PolAir helicopters, paramedics and State Emergency Crews who scoured the banks and water.

Police divers found the four-year-old boy's body just before 3pm, a short distance downstream from the boat ramp.

The vehicle and the boat remain in the water.

The fast current and murky water hindered the efforts of emergency services, police said.

"It is believed the vehicle has been pushed downstream by a strong current," police said in a statement.

A report will be prepared for the Coroner.